FIG. 1 is a simplified view of a representative wireless local area network (WAN) which includes multiple access points (AP1, AP2 and AP3) that provide access to a wired network or LAN 2 through a switch 3. The access points are arranged for overlapped geographic coverage of the WAN, and each access point typically uses a communication channel independent and distinct from the communication channels of other access points in the WAN.
A mobile device 4 operates within the coverage area of the WAN. With respect to the needs of an end user, the primary responsibility of mobile device 4 is to exchange data wirelessly with an access point (such as AP2 in FIG. 1) over the channel on which the access point is operating.
From the perspective of mobile device 4, however, the mobile device has additional responsibilities. Primary among these is to maintain good communications as the mobile device moves around the coverage area of the WAN. For example, as mobile device 4 moves out of the coverage area of AP2, as depicted by the dotted arrow in FIG. 1, toward the coverage area of AP3, mobile device 4 must initiate appropriate hand-off procedures so as to continue to exchange data over the WAN even though data communications has switched from communications with AP2 over a first channel to communications with AP3 over a second channel.
A typical hand-off procedure involves three phases: a discovery phase, a search phase, and an execution phase. Briefly, in the discovery phase, mobile device 4 monitors the quality of communications with its current access point, to determine whether to enter into the search and execution phases. In the search phase, mobile device 4 searches for the availability of other access points. The search phase might be accomplished passively (for example, by listening for broadcasts on other channels), actively (for example, by exchanging probes with access points on other channels), or through receipt of a site report from the current access point (pursuant to IEEE 802.11(k)). In the execution phase, mobile device 4 decides whether other access points would provide better communications than the current access point, and switches communication to the other access point if appropriate.
These activities detract from the ability of mobile device 4 to engage in data exchange with its current access point. For example, during the search phase, if a passive search is performed, mobile device 4 temporarily halts data exchange with its current access point, switches to another channel, and listens on the other channel for beacons and other identifying information from other access points. Typically, this listening time might be on the order of around 100 msec for each other channel. Accordingly, it is possible for many tenths of a second to be spent in passive listening, and many tenths of a second to pass before mobile device 4 returns to its data exchange responsibilities.